Presentation on theme: "Chapter 2: Noise and Vibration"— Presentation transcript:
1Chapter 2: Noise and Vibration | School of Environmental EngineeringEAT 342: Noise Pollution ControlChapter 2: Noise and VibrationBy:Dr. Sara Yasina Yusuf1
2Course Objectives & Learning Objectives This chapter reflects CO2:Ability of defining the properties of sound, quantifying the noise levels and decibel as well as to characterize the noise.Learning Objectives for the chapter:LO-1: Able to SKETCH pure tone wave and DEFINE the properties of sound waves (i.e. Frequency, amplitude, wavelength and period). CALCULATE the speed of sound, sound pressure, frequency and wavelength.LO-2: DEFINE sound scale in decibel.LO-3: DIFFERENTIATE and CALCULATE the Sound Power Level, Sound Intensity and Sound Pressure Level.LO-4: CALCULATE the Sound Pressure/Power Level, summation of sound pressure using formula, table and graph, substraction and averaging of Sound Pressure/Power LevelLO-5: DISCUSS and ANALYZE noise characteristics, such as weighting networks, octave bands and rating systemsLO-6: DISCUSS the techniques in measuring community/environmental noises. COLLECT and ASSESS the Leq as well as Ldn measured through the PBL assignment.LO-7: DISCUSS various types of community noise sources and its criteria. COMPARE the measured data obtained via PBL and CRITIC on the levels to protect human health and welfare
3Content The basic physics of sound Noise measurement Sound and types of soundSpeed of soundCommunity Noise Sources and CriteriaSound pressureProperties of Sound WavesTransportation noiseFrequencyOther internal combustion enginesWavelengthConstruction noiseZoning and siting considerationsCharacteristics of noise and Desibel scaleLevels to protect Health and WelfareFrequency and LoudnessSound level and decibel scaleWeighting networksOctave bandsRating systems
4The basic physics of sound – Sound & Types of Sound What is sound wave?A sound wave is an air pressure disturbance that results from vibration that propagates through an elastic medium (air, water, etc.) at a speed characteristic of that medium.Patterns of noise:Steady-state or continuousIntermittentImpulse or impact
5The basic physics of sound – Sound & Types of Sound Continuous noise is an uninterrupted sound level that varies less than 5 dB during the period of observationIntermittent noise is a continuous noise that persists for more than 1 second that is interrupted for more than 1 secondImpulse noise is a change of sound pressure of 40 dB or more within 0.5 second with a duration of less than 1 second high pitch or intensity, lifetime of less than 1 sec.
6The basic physics of sound - Sound & Types of Sound Behaviour of sound wavesReflectionWaves bounce off a surfaceEchoes & reverberationRefractionWaves bend when they pass through a boundaryDiffractionWaves spread out when they pass through a small gapInterference2 waves superpose to form a resultant wave of greater or lower amplitude
7The basic physics of sound – Sound & Types of Sound ReflectionInterference
8The basic physics of sound – Sound & Types of Sound Example 2.0A fishing boat received the echo 50 ms after sending it.The speed of sound in water is 1500 m/sDetermine the depth of the water.
9The basic physics of sound – Speed of Sound In a free field, sound propagates with the velocity c defined byFor the velocity of sound in air sufficiently accurate at normal temperatures, 0–30oCEqn. 2.1Eqn. 2.2where TK and TR are the temperature in Kelvin and Rankine, respectivelyEqn. 2.3where TC is the temperature in centigrade0F=R - 459
10The basic physics of sound – Speed of Sound Example 2.1Determine the speed of sound at 20oC (68oF) in both metric (m/s) and English (ft/s) units.0F=R - 459
11The basic physics of sound – Sound Pressure For a pure tone, the sound pressure p can be described aswhere a is the amplitude in Pascals, ω is the angular frequency in radians per second, t is the time in seconds, and f is the frequency in hertz.Eqn. 2.4Eqn. 2.5A pure tone is a tone with a sinusoidal waveform, i.e. a sine or cosine wave
12The basic physics of sound – Sound Pressure Figure 2.1, and 2.2 show the pure tone oscillation, although pure tones do not often exist in nature.The field of acoustics and noise control has nearly uniformly adopted the metric system throughout and, as such, the unit used for measuring sound pressure is the Pascal(Pa).
13The basic physics of sound – Properties of Sound Waves Figure 2.1 : Pure tone
14The basic physics of sound – Properties of Sound Waves Figure 2.2: Pure tone from a tuning fork
15The basic physics of sound – Properties of Sound Waves The frequency of a sound indicates the number of cycles performed in 1 s:where T is the period of one full cycle.The unit for frequency is the hertz (Hz):A high frequency pure tone is perceived to have a high pitch. The audible frequency range to humans is 20–20,000 HzAbove 20,000 Hz is ultrasonic. Below 20 Hz is sometimes called infrasonic.Eqn. 2.6
16The basic physics of sound – Properties of Sound Waves Figure 2.3 : Frequency and amplitude
17The basic physics of sound – Properties of Sound Waves
18The basic physics of sound – Properties of Sound Waves The wavelength λ is equal to the distance the oscillations have propagated in the time period T:This shows that the wavelength is inversely proportional to the frequency.In the audio frequency range, the low frequencies have wavelengths of several meters (or feet), whereas the wavelengths for the high frequencies are only a few centimeters (or fractions of an inch).Eqn. 2.7
19The basic physics of sound – Properties of Sound Waves Figure 2.5 : Harmonic oscillation of pressure
20Characteristics of Noise – Frequency and Loudness There are two important characteristics of sound or noise - frequency and loudness.The number of pressure variations per second is called the frequency of sound, and is measured in Hertz (Hz) which is defined as cycles per second.The higher the frequency, the more high-pitched a sound is perceived.Frequency PitchAmplitude LoudnessNo oscillations?
21Characteristics of Noise – Frequency and Loudness Example 2.2Determine the wavelength of a 125-Hz and an 8000-Hz tone at 20oC (68oF) in both metric and English units.
22Characteristics of Noise – Frequency and Loudness The curves in Figure 2.6 indicates the loudness – the subjective interpretation of the magnitude of sound – for pure tones.The loudness of a sound depends on the wave's amplitude.The louder the sound, the higher the amplitude. So, amplitude is also a way of measuring the energy has.The higher the energy, the higher the amplitude resulting a louder sound.
23Characteristics of Noise – Frequency and Loudness 20HzFigure 2.6 : Normal equal loudness contours for pure tones.
24Characteristics of Noise – Frequency and Loudness For example, as can be seen in Fig. 2.6, the threshold of hearing at Hz is about 4 dB (5 x 10-5 Pa).A 20-Hz tone must have a sound pressure level about 70 dB higher than a tone at 1000 Hz in order for a person just to hear the tone.The curves also indicated the loudness—the subjective interpretation of the magnitude of sound—for pure tones.The units describing loudness are called phons. By definition, the phon is equal to the sound pressure level (in dB) reference to 20 μPa of an equally loud 1000-Hz tone.Pain will occur when the loudness exceeds 120 phons.
25Characteristics of Noise – Sound Level and Decibel Scale As explained previously, sound is measured in unit of pressure and normally referred in Pascal and Newton per meter square or “psi”.The lowest audible sound pressure is Pa (or 20x10-6 Pa equivalent 20 μPa).This value is very low as compared to sound emitted from a taking off jet i.e., 200 Pa. Note that the range of difference between these is so great.Therefore, a scale based on the logarithm of the ratios of the measured quantities is used.Measurements on this scale are called levels.
26Characteristics of Noise – Sound Level and Decibel Scale rms Sound PressurePhysically, the rms value is indicative of the energy density of the disturbance.Mathematically, the rms value is obtained by squaring the sound pressures at any instant of time and then integrating over the sample time and averaging the results.The rms value is then the square root of this time average:Where the overbar refers to the time-weighted average and T is the time period of the measurementEqn. 2.8
27Characteristics of Noise – Sound Level and Decibel Scale Sound PowerTravelling waves of sound pressure transmit energy in the direction of propagation of waves magnitude of the displacement times component of force in the direction of the displacementThe rate at which this work is doneSound Power Level is defined as :Where,Lw = Sound Power level in dBW = Sound Power in wattWo = Reference sound power= wattThe standard reference power watt is the threshold power of our hearing.= WORK= (SOUND) POWER, WattEqn. 2.9
28Characteristics of Noise – Sound Level and Decibel Scale Please note that, direct reading of sound power is not possible. Sound Level meter functions by measuring sound pressure or the difference in pressure due to vibration of air molecule, compared to 1 atm.Equation 2.2 describes the power emitted from a noise source.For example, when we speak, our voice vibrates the air molecule causing the pressure to increase and a sound power is generated.What is measured by the instrument is the pressure.Development of sound measurement instruments are currently based on the measurement of pressure and not the power.
29Characteristics of Noise – Sound Level and Decibel Scale Example 2.3Given Lw = 90 dB. What is the sound power in watt?Wo = Reference sound power= wattEqn. 2.10
30Characteristics of Noise – Sound Level and Decibel Scale Sound IntensitySound intensity (I) is defined as the time-weighted average sound power per unit area normal to the direction of propagation of the sound waveI = Sound Intensity (watt/m2)W = Sound Power (watt)A = area (sphere) normal to source (m2)= 4πr2, r is distance from sourceThe higher the area, the lower will be the sound heard at a distance from origin.Therefore, sound intensity is reduced proportionately with increase in coverage area.Eqn. 2.11
31Characteristics of Noise – Sound Level and Decibel Scale Intensity is related to sound pressure in the following manner:Where,I = Intensity (watt/m2)prms = root mean square pressure, Paρ = density of medium (kg/m3)c = speed of sound in medium (m/s)Eqn. 2.12
32Characteristics of Noise – Sound Level and Decibel Scale Both the density of air and speed of sound are a function of temperature. Given the temperature and the pressure, the density of air may be determined from Standard Table.Sound Intensity Level can be written as:Where,LI = Sound Intensity Level (dB)I = Sound Intensity (watt/m2)Eqn. 2.13
33Characteristics of Noise – Sound Level and Decibel Scale Example 2.4Given that a sound power in watt from a pile driver in a construction site is 1x10-3 watt. Determine the sound intensity at the perimeter which will be heard by the following two cases:i) Heidi stands at a distance 15 meter from sourceii) A hawker stands at a distance 50 meter from sourceSound Intensity level?A = 4πr2, r is distance from source
34Characteristics of Noise – Sound Level and Decibel Scale Sound Pressure LevelIn order to cope with the problem of an ‘astronomical’ range of numbers of the sound pressure level (i.e; normal healthy level of Pa vs. Saturn rocket at liftoff of >200 Pa), a scale based on the logarithm is introduced as “levels”The unit for these types of measurement scales is the Bel, named after Alexander Graham Bell:L’ = levels, BelQ = measured quantityQ0 = reference quantityL’ = log10 (Q/Qo) unit: BelEqn. 2.14
35Characteristics of Noise – Sound Level and Decibel Scale The sound pressure level then is a logarithmic ratio Lp defined as:whereprms = the sound pressure of interest (in Pa) andpref = reference sound pressure (in Pa) usually chosen as the limit of hearing of 20 μPa.NOTE: (P log10 xn) = P (n) log10 xThe unit for the sound pressure level, SPL or Lp, is the decibel (dB)Eqn. 2.15
36Characteristics of Noise – Sound Level and Decibel Scale The LP is measured against a standard reference pressure, pref = po = 2 x 10-5 N/m2 which is equivalent to zero decibels.The relationship between sound pressure and sound pressure level (with 20 μPa as the reference sound pressure) is shown in TableA scale showing some common sound pressure level is shown in Figure 2.7.
37Characteristics of Noise – Sound Level and Decibel Scale Table 2.1
38Characteristics of Noise – Sound Level and Decibel Scale Figure 2.7: Relative scales of sound pressure levels
39Characteristics of Noise – Sound Level and Decibel Scale Example 2.5Determine the sound pressure level for sound pressures of p = 1 Pa and p = 1 atm (1.013 × 105 Pa) (reference to 20 μPa)
40Characteristics of Noise – Sound Level and Decibel Scale Combining Sound Pressure LevelsSince we’re dealing with the logarithmic heritage in SPL, adding the decibels is the same as multiplying them.For example, adding 0dB (20µPa) noise with 0dB to it, you’ll get a dB noise.Two approaches: 1) addition through formula, 2) addition through graphical solution
41Characteristics of Noise – Sound Level and Decibel Scale For skeptics, this can be demonstrated by converting the dB to SPL, adding them and converting back to dB.Thus, the addition of these Sound Pressure Level is denoted by:Lp = 20 log10 (P/Po) dBEqn. 2.16Lpt = 10 log10 [ Σ (10)Lpi/10 ] dBEqn. 2.17
42Characteristics of Noise – Sound Level and Decibel Scale The addition of these Sound Power Level is denoted by:Lw = 10 log10 (w/wo) dBEqn. 2.18Lwt = 10 log10 [ Σ 10(Lwi/10) ] dBEqn. 2.19
43Characteristics of Noise – Sound Level and Decibel Scale A graphical solution for this type of problem is provided as in FigureFor noise pollution work, results should be reported to the nearest whole number.
44Characteristics of Noise – Sound Level and Decibel Scale For equal decibel values, a shortcut method can be applied:n10 log10 (n)10.0067.7823.0178.4534.7789.0346.0299.5456.991010.00
45Characteristics of Noise – Sound Level and Decibel Scale Figure 2.8: Graph for solving decibel addition problems
46Characteristics of Noise – Sound Level and Decibel Scale Example 2.6Three SPL’s 68 db, 79 dB and 75 dB, what is SPL of combination?68 dB75 dB= 775.8 dB80.7dB79 dB= 3.2
47Characteristics of Noise – Sound Level and Decibel Scale Alternatively, this can be solved by converting the readings to SPL, adding them and convert back to SPL:
48Characteristics of Noise – Sound Level and Decibel Scale Calculate the final sound power level that would be heard for noise levels 92, 98, 100, 95 and 85 dB using:FormulaGraph
49Characteristics of Noise – Sound Level and Decibel Scale Averaging SoundAverage sound can be calculated as the same as the calculation of summation of sound.As sound is referred in form of log, so the average requires calculation in form of pressure and power. The pressure is then calculated, averaged and finally antilog.
50Characteristics of Noise – Sound Level and Decibel Scale Average Sound Pressure Level is given as:Where,= Average Sound Pressure Level at reference pressure 20 μPa,dB (A)N = Number of sampleLj = Sound Pressure level measured at reference pressure 20 μPa,dB(A)j = 1,2,3…nEqn (a)
51Characteristics of Noise – Sound Level and Decibel Scale The latter equation is only applicable to sound levels in dBAIt may also be used to compute average sound power levels if the factors of 20 are replaced with 10 sWhere,= Average Sound Pressure Level at reference pressure 20 μPa,dB (A)N = Number of sampleLj = Sound power level measured at reference power level W,dB(A)j = 1,2,3…nEqn (b)
52Characteristics of Noise – Sound Level and Decibel Scale Example 2.7Average the Sound Pressure Level for following field monitoring data, 51, 38, 78 and 68 dB(A).Eqn (a)
53Characteristics of Noise – Weighting Networks Weighting networks are used to account for the frequency of a sound.They are electronic filtering circuits built into the sound level meter to attenuate certain frequencies – with a prejudice something like that of the human ear.Normally, there are 3 weighting characteristics: A, B and CThe very low frequencies are filtered quite severely by the A network, in a manner similar to the response of the ear, but only moderately by the B network and hardly at all by the C network. Therefore, if the measured sound level on the C network is much higher than that on the A network, much of the sound energy is concentrated in the low frequency region.
54Characteristics of Noise – Weighting Networks “A” ScaleFilters out low frequenciesResponse curve is similar to sensitivity of human ear“C” ScaleFilters out very little (only the extreme low frequencies)If a measurement is higher on the C scale than the A scale, the noise has a low frequency componentUsed to estimate the effectiveness of ear protectorsA specialized filter, the "D" weighting, has also been introduced for aircraft noise measurements.Figure 2.9 shows the response characteristics of the three basic networks as prescribed by the American National Standards Institute (ANSI) spec. no. S1.4 – 1971.
55Characteristics of Noise – Weighting Networks Figure 2.9: Frequency Response Characteristics of Various Weighting Networks
56Characteristics of Noise – Weighting Networks When a weighting network is used, the sound level meter is electronically subtracts or adds the number of dB shown at each frequency shown in Table 2.2Readings taken when a network is in use are said to be “sound levels” rather than “sound pressure levels”.The readings taken are designated in decibels in one of the following forms: dB(A), dBa, dBA; dB(B), dBb, dBB; dB(C), dBc, dBC. Tabular notations may refer to LA , LB , LC
57Characteristics of Noise – Weighting Networks Table 2.2: Sound Level Meter network weighting values - CFA
58Characteristics of Noise – Weighting Networks Example 2.8A new type 2 sound level meter is to be tested with two pure tone sources that emit 90 dB. The two sources are at 1,000 Hz and Hz. Estimate the expected readings on the A, B and C weighting networks.
59Characteristics of Noise – Weighting Networks Example 2.9The following sound levels were measured on the A, B, and C weighting networks:Source 1: 94 dB(A), 95 dB(B) and 96 dB(C)Source 2: 74 dB(A), 83 dB(B) and 90 dB(C)Characterize the sources as “low frequency” or “mid/high frequency”.
60Characteristics of Noise – Weighting Networks Added Problem The measured octave band sound pressure levels around a punch press is given in table below. Determine the A-weighted sound level and the overall sound pressure level.Octave band center frequency, Hz31.5631252505001000200040008000LP (dB)70818910110393837774CFA, dBLP + CFA (dBA)
61Characteristics of Noise – Octave Bands The human ear is sensitive to sound in the frequency range from approximately 16 Hz to 16 kHz.Impractical to measure the sound pressure level at each frequency in this rangeThe measurements are made over an interval of frequency which is called the bandwidth and is specified by an upper and lower frequency limitfi+1 and fi are called cut-off frequencies.In acoustics the frequency bandwidths are usually specified in terms of octaves and one-third-octavesNormally, considering an 8 to 11 octave bands
62Characteristics of Noise – Octave Bands An octave is defined as an interval of frequency such that the upper frequency limit is twice the lower limit, that is:For the 1/3-octave bands, it is defined as:The center frequency of the band is defined as the geometric mean of the upper and lower frequencies for the interval:Relationship of center frequency for an octave? For 1/3 octave?
63Characteristics of Noise – Octave Bands Generation law for octave and third octave bandsOctave band − oct. filter1/3 Octave band − third oct. filter
64Characteristics of Noise – Octave Bands Table 2.3: Octave bands
65Characteristics of Noise – Octave Bands Added Problem Find the geometric mean frequency for 1:1 octave and 1:3 octave bands for the following band no.Octave Band1/3 Octave BandLowerCenterUpperFrequencyf1 (Hz)f0 (Hz)f2 (Hz) 22 44 88 177
66Characteristics of Noise – Octave Bands Figure 2.10: (a) One-third octave band analysis of a small electric motor. (b) Narrowband of analysis of a small electric motor
67Characteristics of Noise – Octave Bands Noise level measured with 1:1 Octave Band Filters
68Characteristics of Noise – Octave Bands Noise level measured with 1:3 Octave Band Filters
69Characteristics of Noise – Rating Systems Goals of Noise-Rating SystemAn ideal noise-rating system is one that allows measurements by sound level meters or analyzers to be summarized succinctly and yet represent noise exposure in a meaningful wayOur response to sound is strongly dependent on the frequency of the soundSignificant factors in annoyance: type of noise & time of day that it occurredIdeal system: a) frequency, b) daytime or nighttime noise, c) capable of describing the cumulative noise exposure.
70Characteristics of Noise – Rating Systems The LN ConceptLN is a statistical measure that indicates how frequently a particular sound level is exceeded.Example: if L30 = 67 dB, means that 67 dB(A) was exceeded for 30% of the measuring time.A plot of against N (where N = 1%, 2%, 3%,….) look like the cumulative distribution curve (Figure 2.11)Allied to the cumulative distribution curve is the probability distribution curve (Figure 2.12) – showing how often the noise levels fall into certain class intervals.
71Characteristics of Noise – Rating Systems Figure 2.11: Cumulative distribution curve
72Characteristics of Noise – Rating Systems Frequency of occurrence, %Calculation of L30 = 67 dB:Where;N = the sum of the percentagesL = lower limit of the left-most class interval addedEqn. 2.21Figure 2.12: Probability distribution plot
73Characteristics of Noise – Rating Systems The Leq ConceptThe equivalent continuous equal energy level (Leq) can be applied to any fluctuating noise level.It is expressed as:Eqn. 2.22
74Characteristics of Noise – Rating Systems Added Problem Refer to the attachment distributed in the class. Construct a cumulative distribution curve (Sound level vs. Percentage time greater than stated value) Find:LmaxL90LminLeqL1L50
75Characteristics of Noise – Rating Systems Example 2.10Consider the case where a noise level of 90 dBA exists for 10 minutes and is followed by a reduced noise level of 70 dBA for 30 minutes. What is the equivalent continuous equal energy level for the 40-minute period? Assume a five-minutes sampling interval.
76Characteristics of Noise – Rating Systems The Ldn ConceptLdn is the Leq computed over a 24-hr period with a “penalty “ of 10 dBA for a designated night time period.Day-night average – subscript “dn”In airport noise applications, Ldn is referred to LDNNight time period 10 pm to 7 amLdn equation is derived from the Leq equation with the time increment specified as 1 s (1 86, 400 seconds)
77Characteristics of Noise – Rating Systems So, eqn becomes:10 log [1/86400] ≈ 49.4, the day-night average sound level:Eqn. 2.23Eqn. 2.24
78Characteristics of Noise – Rating Systems Time (h)Sound Level (dBA)0000 – 0500520500 – 0700780700 – 1130901130 – 1200701200 – 15301530 – 18001800 – 2200602200 – 0000Example 2.11The USEPA estimated that, in 1974, the following was a typical noise exposure pattern for a factory worker living in an urban area. Estimate the Ldn for the exposure shown.
80Content The basic physics of sound Noise measurement Sound and types of soundSpeed of soundCommunity Noise Sources and CriteriaSound pressureProperties of Sound WavesTransportation noiseFrequencyOther internal combustion enginesWavelengthConstruction noiseZoning and siting considerationsCharacteristics of noise and Desibel scaleLevels to protect Health and WelfareFrequency and LoudnessSound level and decibel scaleWeighting networksOctave bandsRating systems
81Noise Measurement – Measuring noise Noise measurement equipment depends on the task to be performedFor an initial survey – a sound level meter (SLM) is adequate for a rapid evaluation and identification of potential problem areasTo study and also determine the characteristics of a noise problem area – an SLM, frequency analyzer and recorder are needed
82Noise Measurement – Measuring noise Sound Level MeterUsed to measure the sound pressure levelAvailable to cover a range of 20 to 180 dBSpecifications refer to the American National Standards Institute (ANSI) – referred as “Specifications for Sound Level Maters (ANSI S )Weighting networks of A, B, C are provided – total loudness level for a particular situation with consideration of the sound frequency, intensity and impact levels.Weighting network A – most commonly used: discriminates against frequency below 500 Hz – encompasses the most sensitive hearing range.Measuring environmental noise should be supplemented by the time/duration to determine the total quantity of sound affecting people
83Noise Measurement – Measuring noise Sound Level Meter (cont’d)Types of SLM:SLM provides setting for “F” (fast time response) and “S” (slow time response)Calibration by calibrator – generates a known decibel standard for QA/QCSLM TypeIntended Uselaboratory reference standard1for laboratory use, and for field use where the acoustical environment has to be closely specified and controlled2suitable for general field applications3primarily for field noise survey applications
84Noise Measurement – Measuring noise Noise DosimeterMeasure the amount of potentially injurious noise to which an individual is exposed over a period of timeCan be set to the desired level – total up the exposure time to noise above the set levelDoes not identify the noise sourcesTo determine the noise exposure and culpability – dosimeter should be coupled with a frequency analyzer/ human observer to record noise source identities
85Noise Measurement – Measuring noise Noise Dosimeter (cont’d) – Interpretation of resultsTo calculate the noise exposure level of an employee working shifts of more or less than eight hours, it is necessary to normalise the employee’s exposure to an equivalent eight hour exposure (LAeq,8h).where:LAeq equals the equivalent continuous A-weighted sound pressure level occurring over the measured time; andT represents the shift length in hours (not to be confused with the sampling time).For shifts between 10 to 12 hours, add 1 dBA – extended shiftIn addition, shifts of 10 hours or more require adjustments to LAeq,8h values, as indicated in Table 3.1.
86Noise Measurement – Measuring noise Table 3.1Correction factors for computing LAeq,8h from LAeq records
87Noise Measurement – Measuring noise Example 3.1A personal noise dosimeter is placed on an employee for a representative period of six hours. At the end of the six hours, the LAeq reading is 93 dB(A). The employee works a 10 hour shift.
88Noise Measurement – Measuring noise Sound AnalyzerFrequency analyzer – measure complex sound and sound pressure according to frequency distribution.Supplemented with SLMCovers different frequency bandsExample: octave band analyzer, impact noise analyzer (peak level and duration of impact noise)
89Noise Measurement – Measuring noise Cathode-Ray OscillographObserving the wave form of a noise and patternMagnetic tape recorder makes possible the collection of noise information in the field and subsequent analysis of the data in the office or laboratory
90Noise Measurement – Measuring noise Table 3.2: Background noise correction factorBackground NoiseNoise in the absence of the sound being measured that may contribute to and obscure the measured soundCorrection can be made through subtraction method or application of correction factors (CF) as in Table 3.2ΔL, dBAΔ, dB1.06.91.55.32.04.32.53.63.03.52.6ΔL, dBAΔ, dB6.51.17.01.07.50.98.00.79.00.6100.5120.3140.2160.118200.04.02.24.51.95.01.75.51.46.01.3
91Noise Measurement – Measuring noise Example 3.2The measured overall sound pressure level around a fan is 83 dB. The measured overall sound pressure level for the background (ambient) noise in the room where the fan is located is 77 dB. Determine the overall sound pressure level produced by the fan alone.
92Noise Measurement – Measuring noise Added problemThe experimental data shown below were measured around an air vent. The readings are the octave band sound pressure levels with the air flow stopped (background noise) and with the air flowing (data). Determine the octave band sound pressure levels and overall sound pressure level for the vent noise aloneOctave band center frequency, Hz631252505001000200040008000Background LP, dB79.076.173.670.768.166.064.363.0Data LP, dB79.176.676.378.781.283.178.1
93Community Noise Sources and Criteria Estimation of Community ReactionIf noise spectrum data are not available, the LDN of the background noise, with suitable correctors may be used to estimate the anticipated community response to the environmental noise:The correction made to measured Ldn accounted for the effect of annoyance due to several influencing factors (presented in Table 4.1) such as below:NighttimeLocationTime of the yearPrevious noise exposureAverage community reaction to noise based on Ldn is given in Table 4.2.
94Community Noise Sources and Criteria Estimation of Community Reaction – Table 4.1. Correctors to be Added to the Measured Day-Night Level for Various Influencing Factors for Community Noise ReactionbInfluencing FactorDescription of conditionCFdn, dBANoise SpectrumPure tones or impulsive noise present+5No pure tone or impulsive soundsType of locationQuiet suburban or rural community+10Normal suburban communityUrban residential communityNoisy urban residential community-5Very noisy urban community-10Time of yearSummer or year-roundWinter only or windows always closedPrevious noise exposureNo prior experience with the intruding noiseSome prior experience with the noise or where the community is aware that good-faith efforts are being made to control noiseConsiderable experience with the noise and the group associated with the source of noise has good community relationsAware that the noise source is necessary, of limited duration, and/or an emergency situationbOnly one correction factor should be used from each category
95Community Noise Sources and Criteria Estimation of Community Reaction – Table 4.2. Average Community Reaction to Noise Based on the Day-Night Level, (Ldn)Corrected day-night level Ldn(corrected)Expected community response<62 dBA (dn)No reactiondBA (dn)ComplaintsdBA (dn)Threats of community action>72 dBA (dn)Vigorous community action
96Community Noise Sources and Criteria Example 4.1The noise levels in a suburban area are given in Table 4.3. The area has had some prior experience with intrusive noises. There are no pure tone components of the noise, and it is not impulsive. The noise source will be present year-round.Determine the anticipated community response to the noise source.DurationA-weighted levelDaytime4 hours60 dBA6 hours55 dBA5 hours50 dBANighttime2 hours45 dBA7 hours40 dBA
97Community Noise Sources and Criteria – Transportation noise Aircraft NoiseThe noise spectra of a wide fan jet (i.e. Boeing 747) reveal that sound pressure levels are higher on takeoff compared to landingSmaller aircraft have lower sound pressure levels (except for turbojets)Highway Vehicle NoisePredominant source of most automobiles during normal operation below about 55 km/h is the exhaust noiseAt speed 80 km/h, tire noise is dominant source. For truck, at speed more than 80 km/h tire noise is dominant – the noisiest is “cross-bar” treadDiesel trucks are 8 to 10 dB noisier than gasoline-poweredFor motorcycle, dominant source of noise is exhaust – highly dependent on the speedThe U.S. Federal Highway Administration (FHA) has developed standards shown in Table 4.3
98Community Noise Sources and Criteria – Transportation noise Table 4.3. FHA noise standards for new constructiona Either Leq or L10 may be used, but not both. The levels are to be based on a 1-hour sampleLand Use CategoryExterior design noise level dBAaDescription of land use categoryLeqL10A5760Tracts of land in which serenity and quiet are of extraordinary significance and serve an important public need, and where the preservation of those qualities is essential if the area is to continue to serve its intended purpose. For example, such areas could include amphitheaters, particular parks or portions of parks, or open spaces, which are dedicated or recognized by appropriate local officials for activities requiring special qualities of serenity and quietB6770Residences, motels, hotels, public meeting rooms, schools, churches, libraries, hospitals, picnic areas, recreation areas, playgrounds, active sports areas, and parksC7275Developed lands, properties, or activities, not included in categories A and B aboveDUnlimitedUndeveloped landsE52 (interior)55 (interior)Public meeting rooms, school, churches, libraries, hospitals and other such public buildings
99Community Noise Sources and Criteria – Other internal combustion engines These devices as listed in Table 4.4 are not significant to average residential noise levels in urban areaHowever the relative annoyance of most of the equipment tends to be high – U.S. EPA, 1971The 8-hour exposure level is in reference to the equipment operatorTable 4.4. Summary of noise characteristics of internal combustion engines
100Community Noise Sources and Criteria – Construction noise Table 4.5. Range of sound levels from various type of construction equipment (based on limited available data samples. (Source: U. S. EPA, 1972)19 common types of construction equipment – range of sound levels as in Table 4.5.Annoyance resulting from construction noise:Single house construction in suburban communities will generate sporadic complaints if the boundary line 8-hour Leq exceeds 70 dBAMajor excavation and construction in a normal suburban community will generate threats of legal action if the boundary line 8-hour Leq exceeds 85 dBA
101Community Noise Sources and Criteria – Zoning and siting considerations The U.S. Dept. of Housing and Urban Development (HUD) was charged with developing guides for zoning modifications or for siting of dwellingsAnnoyance for a specific noise exposure depended on both the average level of the noise and on the variability of the source of noise (Griffiths and Langdon, 1968)Table 4.6 list out criteria for new residential construction by HUDGeneral external exposuresAssessmentExceeds 89 dBA 60 minutes per 24 hoursUnacceptableExceeds 75 dBA 8 hours per 24 hoursExceeds 65 dBA 8 hours per 24 hoursDiscretionary: normally unacceptableLoud repititive sounds on siteDoes not exceed 65 dBA more than 8 hours per 24 hoursDiscretionary: normally acceptableDoes not exceed 45 dBA more than 30 minutes per 24 hoursAcceptable
102Community Noise Sources and Criteria – Levels to protect Health and Welfare Noise criteria levels that is necessary to protect the health and welfare of U.S. citizens listed in Table 4.7